HTC has just officially announced the Windows Phone version One (M8) - HTC One (M8) for Windows. The Windows Phone version HTC One (M8) is same as the regular Android One (M8).

The difference between the One (M8) for Windows and the original version is just the operating system. The One (M8) for Windows runs on Windows Phone 8.1, obviously, instead of Android. Cortana will be replacing Google Now on this version of M8 as your personal assistant. Thankfully, BlinkFeed is still available on it, but not in the homescreen.
The rest of it are exactly same - 5" 1080p display wrapped in an aluminium body, Snapdragon 801 SoC, 2 GB of RAM, 32 GB expandable storage, 4 MP UltraPixel camera with secondary depth sensor, 5 MP front camera and a ton of connectivity.

The HTC One (M8) for Windows currently is the fastest Windows Phone device (Nokia Lumia 930 only packs a Snapdragon 800 SoC).

The phone is currently available in United States through Verizon Wireless exclusively, expect international version will release soon.